Recently I had an issue where I was using a mini computer to run some Docker services and I kept running out of space on the tiny 16GB hard drive. The hard drive for this machine was soldered on the motherboard so there was no possibility to add a larger drive. Considering Docker itself was using about 9GB of that space, the logical solution was to mount the data directory to another disk, which is this case was a low profile USB 3.1 thumb drive. The steps to complete this are quite simple and here they are:

• First, you need to stop the Docker service:systemctl stop docker.service
• Next, make a new directory somewhere within your mount point and copy the files:
  mkdir -p /path/to/new-docker
rsync -aqxP /var/lib/docker/ /path/to/new-docker

• Open /etc/docker/daemon.json and add the new data directory

  {
    "/path/to/new-docker"
  }

• Now the part noone tells you, make sure the external drive is mounted before the Docker service starts.
  • First, get the system mount unit
    systemctl list-units --type=mount
  • In my case, the USB was mounted to /media/jukebox/Samsung
  • Therefore, the mount unit is seen below is media-jukebox-Samsung.mount
    
  • Open docker service file and paste the mount unit at the end of the line beginning with After
    • Before:
      After=network-online.target firewalld.service containerd.service
    • After edit:
      After=network-online.target firewalld.service containerd.service media-jukebox-Samsung.mount
  • The service will now wait for that volume to be mounted before it starts.
• Finally, Restart Docker
  systemctl start docker.service